Improvements in animal feeds to enable animals to digest the feeds more efficiently are constantly being sought. One of the main concerns is to improve the Feed Conversion Ratio (FCR) of a feed without increasing its cost per unit weight. The FCR of a feed is the ratio of the amount of feed consumed relative to the weight gain of the animal. A low FCR indicates that a given amount of feed results in a growing animal gaining proportionately more weight. This means that the animal is able to utilise the feed more efficiently. One way in which the FCR can be reduced is to improve its digestibility by an animal thereby increasing the nutritional benefit which the animal can derive from it.
There are various constraints on the digestibility of the nutritional components of a feed such as its starch, fat, protein and amino acid content. These constraints include:
(i) the viscosity of materials present in the animal's gut. Such viscosity is due, at least in part, to soluble non-starch polysaccharides such as mixed-linked .beta.-glucans and arabinoxylans; PA1 (ii) entrapment of nutrients within the cell walls of the feed, particularly those of the aleurone layer in cereals. Such entrapment is caused by the high levels of non-starch polysaccharides in the cell walls of cereals which are relatively resistant to break-down by the animal's digestive system. This prevents the nutrients entrapped within the cells from being nutritionally available to the animal; and PA1 (iii) a deficiency in endogenous enzyme activity, both of the animal and of the gut microbial population particularly in a young animal.
The above problems which interfere with digestibility are particularly noticeable in the case of cereal-based diets, such as those having a high wheat content.
Due to the problem of poor digestibility of nutrients from the feed, it is normally necessary to formulate feeds to contain higher levels of energy and protein providing materials in order to meet the nutritional demands of animals.
There is now a substantial body of evidence that incorporating certain (supplementary) enzymes in cereal-based animal feeds can be advantageous in reducing the viscosity of material present in the animal's gut. This reduction can be achieved by enzymes such as xylanases which hydrolyse soluble xylans thereby reducing digesta viscosity which is an important constraint on the process of digestion.
The xylanases which are added as supplements must be stable and active at the pH and temperature conditions found within the gastrointestinal (GI) tract of the target animal. If they are not stable and active when exposed to such in vivo conditions, then they will not be able to reduce digesta viscosity to any significant extent. It is presently known to include xylanases as a supplement in an animal feed derived from fungi such as Trichoderma longibrachiatum, Aspergillus niger and Humicola insolens.
Bedford and Classen (The Journal of Nutrition, vol. 122, pp 560-569) disclose that there is a significant correlation between digesta viscosity measured in vivo in the case of broiler chickens and bodyweight gain and FCR values. In the case of wheat and rye-based diets fed to poultry, it was shown that as much as 70-80% of the variations in the weight gain and FCR are based upon differences in intestinal viscosity alone. This highlights the importance of digesta viscosity in cereal-based feeds containing high levels of soluble arabinoxylans. As digesta viscosity increases, it reduces the digestibility of all nutrients by interfering with the diffusion of pancreatic enzymes, substrates and the end products of the digestion process.
It has been found that the inclusion of a xylanase in an animal feed helps to reduce the digesta viscosity in livestock. As a result of this, the animal's ability to digest the feed is increased, the rate of bodyweight gain of the animal per unit amount of feed consumed is increased, and the FCR of the feed is decreased.
It is conventional to include enzyme supplements, such as xylanase, in an animal feed by impregnating the enzyme onto a physiologically acceptable carrier, such as a cereal. The impregnated carrier is mixed with the other components of the feed and then pressed into cubes or pellets for feeding directly to animals.
There has recently been much development in the processing of the various feed components into forms such as cubes and pellets. The processes which have been developed make use of relatively high temperatures. This is firstly to improve the efficiency of the manufacturing process and secondly to produce feeds which are free from harmful bacteria, particularly Salmonella. In addition, the use of high temperatures improves the quality and durability of the resulting cubes and pellets, increases the range of ingredients which can be efficiently handled and also increases the level of liquid ingredients, such as fat and molasses, which can be incorporated into the feed.
The processing techniques currently employed apply relatively high temperatures to the mixture of feed components for a relatively long period. Further, the mixture is subjected to relatively high pressures during processing which also helps to increase the durability of the cubes or pellets formed.
One of the processing methods which has been developed to improve the nutritional properties of the feed is steam pelleting. This method includes the step of treating the compounded feed with steam to increase its temperature and moisture content. This step is termed conditioning. Conditioning lasts from a few seconds up to several minutes depending on the type and formulation of the feed. The temperature in the conditioner may rise to 100.degree. C. Afterwards, the feed is passed through a pelleting die which causes a rapid increase in its temperature due to friction.
Recently, a new device for pre-treatment or conditioning of feeds has been introduced called an expander. This allows sustained conditioning under pressure followed by pelleting. According to this technique, various feed components which have previously been subjected to steam-conditioning are fed into a compression screw into which more steam is injected, and the mass is then subjected to increasing pressure and shear action and then forced through a variable exit gap. The compressed product, after reduction in particle size, is fed into a standard pelleting press.
The dwell time of the feed components in the expander is about 5-20 seconds, and the temperature reached may be as high as 145.degree. C. A compression pressure of about 3.5 MPa is reached, but the build-up of both temperature and pressure is very quick and both fall rapidly as the product is expelled through the exit gap.
The use of expanders is advantageous because they effectively eliminate harmful bacteria, particularly Salmonella. Furthermore, it is possible to include relatively high levels of fat and other liquid ingredients in the mixture prior to pelleting. In addition, the cooking and pressure/shear action results in greater starch gelatinisation.
Unfortunately, these high temperature and high pressure processing conditions, particularly when applied in the moist conditions normally encountered during pelleting, are potentially destructive to certain feed components. This is particularly true of any enzymes, including xylanases, which are present.
As is well known, enzymes are proteins, and thus are made up of amino acids. The particular sequence of amino acids, the "primary structure", determines the nature of the protein. The amino acid chain may then be arranged in a number of "secondary structures" such as sheets and helices. These structures are also organised in relation to each other to give a "tertiary structure"; for example the sheets may lie parallel to each other, rather like the pages of a newspaper. Lastly, several sub-units may be associated together in a particular enzyme, and this gives rise to "quaternary structure".
To function, an enzyme must possess an active site which is capable of catalysing the reaction of a particular substrate. This active site often has a very specific shape, which is determined by the primary, secondary, tertiary and quaternary structures of the enzyme. Changes in the shape of the catalytic site are likely to deactivate the enzyme.
There are several factors, including heat, pressure and pH, which may alter the shape of an enzyme and so also its active site. During feed processing, any enzyme already present in the mixture will be subjected to temperatures and pressures which may well cause the enzyme to at least partially denature and thus lose some or all of its activity. The higher the temperature to which an enzyme is exposed during processing, the greater its activity will decline. Typically, mesophilic xylanases are stable at temperatures up to 65.degree. C. but lose all activity if exposed to a temperature of 95.degree. C. at least in an aqueous solution.
If such temperature mediated denaturing occurs during feed processing, it is of course extremely disadvantageous as then the enzyme will not give rise to the effect for which it was added, or will only give rise to such an effect to a limited extent. One possibility of overcoming this problem would be to include significantly greater relative amounts of the enzyme to the feed in order to compensate for the deactivation of a certain proportion. However, adding such additional amounts is disadvantageous from an economic viewpoint because the enzymes which are incorporated in animal feeds are relatively expensive.
It has also been investigated to stabilise the enzymes by coating them on special carriers or by coating them using specialised coating technologies. However, methods such as these have not been able to deal effectively with the relatively severe processing conditions encountered in high temperature steam pelleting, in an expander or in an extruder.
An alternative solution would be to add enzymes, such as xylanase, to pre-formed pellets which have already been heat treated. This however is not an ideal solution because firstly complex and expensive machinery is required to precision coat the enzyme on the pellets to achieve the desired relative amount of inclusion.
Secondly, the solutions of enzymes which are used in such a coating procedure have limited storage stability and can become contaminated by bacteria.
Accordingly, even though partial solutions to the problem of enzyme stability during feed processing are available, none of them solves the problem in a totally effective manner.
In the description and claims which follow, reference is made to units of xylanase activity. This activity as used in the present invention is measured by the following assay method.